.TH std::span::at 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::span::at \- std::span::at

.SH Synopsis
   constexpr reference at( size_type pos ) const;  (since C++26)

   Returns a reference to the element at specified location pos, with bounds checking.

   If pos is not within the range of the span, an exception of type std::out_of_range
   is thrown.

.SH Parameters

   pos - position of the element to return

.SH Return value

   Reference to the requested element, i.e. *(data() + pos).

.SH Exceptions

   std::out_of_range if pos >= size().

.SH Complexity

   Constant.

.SH Notes

   Feature-test macro  Value    Std      Feature
   __cpp_lib_span     202311L (C++26) std::span::at

.SH Example


// Run this code

 #include <iostream>
 #include <span>
 #include <stdexcept>

 #ifdef __GNUG__
 [[gnu::noinline]]
 #endif
 unsigned int runtime_six() // Emulate runtime input
 {
     return 6u;
 }

 int main()
 {
     int x[]{1, 2, 4, 5, 5, 6};
     std::span data(x);

     // Set element 1
     data.at(1) = 88;

     // Read element 2
     std::cout << "Element at index 2 has value " << data.at(2) << '\\n';

     std::cout << "data size = " << data.size() << '\\n';

     try
     {
         // Set element 6, where the index is determined at runtime
         data.at(runtime_six()) = 666;
     }
     catch (std::out_of_range const& exc)
     {
         std::cout << exc.what() << '\\n';
     }

     // Print final values
     std::cout << "data:";
     for (int elem : data)
         std::cout << ' ' << elem;
     std::cout << '\\n';
 }

.SH Possible output:

 Element at index 2 has value 4
 data size = 6
 std::out_of_range: pos >= size()
 data: 1 88 4 5 5 6

.SH See also

   operator[] access specified element
              \fI(public member function)\fP
